The invention described concerns a heat and moisture exchange media and particularly an improved fibrous media in the form of a nonwoven web.
A number of different approaches to heat and moisture exchange media, media arrangement, and housing designs have been proposed in the art. For example, U.S. Pat. No. 4,090,513 (Togawa) proposes a heat/moisture exchange device for attachment to a cannula inserted into the trachea. The heat/moisture exchange media comprises alternating layers of heat-conductive sheets (aluminum mesh) and heat-insulating sheets (gauze or a nonwoven fabric). It is stressed that the sheets should be perpendicular to the passage of air to create air layers isolating the heat-insulating layers to sharpen the temperature gradient across the body. A disadvantage with this media and the media arrangement design is the increase in the mass of the media and its resulting dead space.
Another alternating layer media is described in U.S. Pat. No. 4,771,770 (Artemenko et al.), directed to a heat/moisture exchange device used in a regeneration-type breathing apparatus. The heat/moisture exchange material comprises alternating layers of a nonwoven hydrophilic material impregnated, with a hygroscopic substance, with adjacent lower-density hydrophobic webs. Air distribution means are provided on either face of the body to direct more air to the peripheral edges of the medium. This device is bulky and the hygroscopic materials described are water soluble, which can be ingested by returned condensed water vapor.
Other patents also describe the use of media impregnated with water soluble hygroscopic material. Canadian Patent No. 1 259 869 describes a heat/moisture exchange medium comprised of a corrugated paper product impregnated with a hygroscopic material such as lithium chloride, glycols, or potassium chloride or acrylic acid, glycerin (used in antifreeze, a carcinogen) being most preferred. The corrugations are stacked, as opposed to a roll form, stated as providing a more non-uniform pressure drop and fluid flow, referring to several prior Japanese applications (see also U.S. Pat. No. 5,022,394). U.S. Pat. No. 5,042,468 (Lambert) describes a specific breathing device using a heat/moisture exchange medium comprising spirally-wound strips of corrugated cardboard impregnated with hygroscopic material or other conventional media. U.S. Pat. No. 4,516,573 (Gedeon) describes a heat/moisture exchange device, the media comprising polypropylene nonwovens impregnated with conventional hygroscopic substances. Problems again can arise due to the water solubility of the hygroscopic material, which can be subsequently returned to the patient. The water-soluble hygroscopic material also requires impregnation of the carrier which can complicate manufacturability.
EPA No. 265,163 describes a heat/moisture exchange device for attachment to tracheal or tracheostomy tubes. The material disclosed contains a layer of electrostatically charged polypropylene and a second layer of a pre-treated hydrophilic open-cell polyurethane foam able to absorb approximately 30 times its weight in moisture. Both layers act to condense moisture. U.S. Pat. No. 3,881,482 (Lindholm) describes a heat/moisture exchange device, the media comprising a soft open-celled plastic such as polyurethane or polyethylene foam. These foam layers would generally exhibit excessive pressure drops.
U.S. Pat. No. 5,035,236 (Kanegaonkar) describes a device comprising a corrugated material of glass fiber; the glass fiber media is transverse to the direction of air flow. The primary purpose of this media appears to be as a filter although the patent also notes that it has some heat/moisture exchange capabilities.
U.S. Pat. No. 4,327,717 (Oetjen et al.) describes an unusual heat/moisture exchange device where the heat/moisture exchange is conducted in parallel, hollow fibers of a permeable diaphragm material coated on one surface with copper or silver by cathode sputtering. This approach would be exceedingly costly and difficult to manufacture.